The Sampler of the Videopulse Georadar
The given work is devoted to the theoretical analysis of noise characteristics and distortions of the sampler with the bandwidth 1 GHz and sensitivity 1 mV made according to the bridge diode circuit. The results of test measurements of the sampler working setup are given in the work.
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irk-123456789-1223522017-07-03T03:03:27Z The Sampler of the Videopulse Georadar Kholod, P.V. Ruban, V.P. The given work is devoted to the theoretical analysis of noise characteristics and distortions of the sampler with the bandwidth 1 GHz and sensitivity 1 mV made according to the bridge diode circuit. The results of test measurements of the sampler working setup are given in the work. В работе представлен теоретический анализ шумовых характеристик и искажений стробоскопического преобразователя с полосой пропускания 1 ГГц и чувствительностью 1 мВ, построенного по мостовой диодной схеме. Приведены результаты тестовых измерений макета преобразователя. У роботі представлений теоретичний аналіз шумових характеристик та спотворень стробоскопічного перетворювача з полосою пропускання 1 ГГц та чутливістю 1 мВ, побудованого за мостовою діодною схемою. Наведено результати тестових вимірювань, проведених за допомогою макета перетворювача. 2002 Article The Sampler of the Videopulse Georadar / P.V. Kholod, V.P. Ruban // Радиофизика и радиоастрономия. — 2002. — Т. 7, № 4. — С. 424-430. — Бібліогр.: 15 назв. — англ. 1027-9636 http://dspace.nbuv.gov.ua/handle/123456789/122352 en Радиофизика и радиоастрономия Радіоастрономічний інститут НАН України |
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The given work is devoted to the theoretical analysis of noise characteristics and distortions of the sampler with the bandwidth 1 GHz and sensitivity 1 mV made according to the bridge diode circuit. The results of test measurements of the sampler working setup are given in the work. |
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Kholod, P.V. Ruban, V.P. |
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Kholod, P.V. Ruban, V.P. The Sampler of the Videopulse Georadar Радиофизика и радиоастрономия |
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Kholod, P.V. Ruban, V.P. |
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Kholod, P.V. |
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The Sampler of the Videopulse Georadar |
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The Sampler of the Videopulse Georadar |
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The Sampler of the Videopulse Georadar |
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The Sampler of the Videopulse Georadar |
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The Sampler of the Videopulse Georadar |
| title_sort |
sampler of the videopulse georadar |
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Радіоастрономічний інститут НАН України |
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The Sampler of the Videopulse Georadar / P.V. Kholod, V.P. Ruban // Радиофизика и радиоастрономия. — 2002. — Т. 7, № 4. — С. 424-430. — Бібліогр.: 15 назв. — англ. |
| series |
Радиофизика и радиоастрономия |
| work_keys_str_mv |
AT kholodpv thesamplerofthevideopulsegeoradar AT rubanvp thesamplerofthevideopulsegeoradar AT kholodpv samplerofthevideopulsegeoradar AT rubanvp samplerofthevideopulsegeoradar |
| first_indexed |
2025-07-08T21:33:41Z |
| last_indexed |
2025-07-08T21:33:41Z |
| _version_ |
1837116084481687552 |
| fulltext |
Radio Physics and Radio Astronomy, 2002, v. 7, No. 4, pp. 424-430
THE SAMPLER OF THE VIDEOPULSE GEORADAR
P.V. Kholod, V.P. Ruban
A.Ya. Usikov Institute for Radiophysics and Electronics of the NAS of Ukraine,
12, Акad. Proscura St., 61085, Kharkov, Ukraine
E-mail: kholod@ire.kharkov.ua
The given work is devoted to the theoretical analysis of noise characteristics and distortions of the sampler
with the bandwidth 1 GHz and sensitivity 1 mV made according to the bridge diode circuit. The results of test
measurements of the sampler working setup are given in the work.
1. Introduction
The modern approach to the practical solution of
problems of videopulse subsurface sounding requires
improvement of the technique of receiving and proc-
essing the sounding signals of nano- and subnano-
second duration. The most important problem is the
problem of precise determination of time and ampli-
tude parameters of the wideband sounding signal.
The measurement accuracy of these characteristics is
one of the most important factors influencing the
operation efficiency of the whole radar system.
For precise determination of time-amplitude
characteristics of videopulse signals of nanosecond
duration radiated by georadars it is necessary to use
ultrawideband samplers with the bandwidth 1 GHz
providing conversion of sounding signals with the
rise (decay) time of the order of 1 ns.
Generally high-speed samplers are made accord-
ing to the classical circuit without feedback [1] and
include the following functional parts: the switch
circuit (sampling gate), strobe generator (sampling
gate control section), stretcher (storage capacitor)
with the buffer repeater and compensating and ampli-
fying circuits.
At present samplers with high frequency transis-
tors as elements of the switch circuit are widely used
in radar equipment. But such circuits do not satisfy
requirements to up-to-date videopulse georadars both
in bandwidth and dynamic [1].
As it is know from [1-5] it is possible to extend
the bandwidth and simultaneously the sampler dy-
namic range by using the sampling gate with high-
speed diodes.
From the known sampling gate circuits [4, 6-9]
such as: one-diode circuits, balanced two-diode cir-
cuits, balanced bridge circuits and transistor circuits
in the sampler with the bandwidth 1 GHz it is more
preferable to use the sampling gate bridge circuit. Its
merits are the high input resistance eliminating the
influence on a previous cascade, small signal distor-
tions during conversion, the high sensitivity and sta-
bility. The circuit demerit is the complexity of identi-
cal diode pairs matching. However, diode bridges are
produced in lots at present and it simplifies the prob-
lem.
The following strobe generator circuits are well-
known: circuits with tunnel diodes, with the Gunn
diodes, with charge-storage diodes with series and
parallel connection, and also avalanche transistor
circuits [1, 10-14]. From the specified circuits the
circuit with parallel-connected charge-storage diodes
is the most suitable as a strobe generator for the sam-
pler with the balanced bridge circuit of the sampling
gate. The merit of this circuit is the simplicity of the
short voltage step forming and power-supply noise
compensation due to the specific connection of the
output transformer.
The buffer repeater is the most suitable as a
stretcher for the sampler. The given repeater exerts
the smallest influence on signal stretching and is well
matched with following cascades.
The results of development of the high-speed
sampler with the sampling gate built according to the
bridge circuit, the strobe generator with series-
connected charge-storage diodes and voltage
stretcher with the buffer repeater.
2. Noise and Stray Parameters of the
Sampling Gate
The measuring system sensitivity and boundary
bandwidth are determined by the internal noise level.
So, first of all, it is necessary to determine the noise
level at the sampler output. The sampler circuit is
shown in Fig. 1.
The energy spectrum of noise sources is de-
scribed by the expression [6]
2
2 4f rev
eq rev eq eq
A I
w R qI R kTR
ω
= + + , (1)
The Sampler of the Videopulse Georadar
Radio Physics and Radio Astronomy, 2002, v. 7, No. 4 425
where in
eq
in
R RR
R R
∂
∂
=
+
, R∂ is the differential resis-
tance of the non-conducting diode, inR is the input
resistance of the buffer amplifier, fA is the flicker
noise factor, q is the elementary charge, k is the
Boltzmann constant, ω is the frequency, revI is the
diode reverse current, T is the equivalent noise tem-
perature. At that the noise-voltage dispersion is:
2
2 2
1
2 2
ln 1
2 4 ,
f rev
u eq
rev eq eq
A I
R
qI R kTR
ωσ
π ω
ω ω
∆ = + +
∆ + ∆ (2)
where ω∆ is the frequency band, in which the noise
is calculated, 1ω is the lower bound of the band-
width.
In the second members of the formulas (1) and
(2) the first term describes the diode flicker noise, the
second one – the shot noise, and the last one – the
thermal resistance noise. Due to the fact that the cir-
cuit with balanced diode inclusion is used in the
sampler the thermal noise (30 Vµ ), noise of the
sampling gate input load and also charge-storage
diode noise contribute in a smaller way than the noise
of the sampling gate diodes. Therefore, the shot and
flicker noise of diodes is the most essential for the
bridge circuit. Their amplitude at the sampler output
amounts to 340 Vµ . Consequently, the resultant
noise level of the sampler doesn’t exceed 341 Vµ .
The waveform transfer accuracy by the sampler
is determined by the level of distortions appearing
due to the influence of the stray inductive and capaci-
tive diode parameters. These parameters appear in
the equations (3) and (4) describing the time conver-
sion of a signal in the following way:
( )
( )
( )
( ) ( )
( )
for
for
2
1 1 1 12
2
1 1 1 1 12
1
0;
0;
sp
sp
sp
sp
d u du
LC RC u u t E
dtdt
E u t E
d u dF duLC RC L u
du dtdt
R F u u t E
u t E
+ + = −
− ≤ − <
+ + + +
+ = −
− ≥
(3)
( )
( )( )
( )
( )
( )
for
for
2
2 2 2 22
1
2
2 2 2 2 22
2 2
1 2
,
1 ,
s
s
d v dv
L C RC u u t
dtdt
t t
d v dvL C RC L g t
dtdt
dg t
v R g t L u t
dt
t t t
+ + = Θ
<
+ + +
+ + − = Θ
≤ ≤
(4)
where ( )F u is the equation of voltage-current char-
acteristic of series-connected diode junctions,
1 SG DL L L= + , SGL is the inductance of the
Fig. 1. The sampler circuit
P.V. Kholod, V.P. Ruban
426 Radio Physics and Radio Astronomy, 2002, v. 7, No. 4
strobe generator output, DL is the inductance of the
sampling gate diode leads, 1 SG BR R R= + , SGR is
the strobe generator resistance, BR is the diode base
resistance, 1C is the diode capacitance including the
average junction capacitance DC , ( )spu t is the time
dependence of the strobe pulse voltage, E is the
cutoff voltage of the sampling gate diodes, 1t , 2t is
time of diode opening and closing, ( ),su tΘ is the
expression describing the signal in the coordinate
system, in which the time coordinate origin coincides
with the strobe pulse arrival time [5],
( )
( )dF ug t
du
= is the time dependence of the diode
differential conductance, where u is the solution to
the equation (3), 2 S DL L L= + , SL is the induc-
tance of the signal generator output,
2 S BR R R= + , and SR is the signal generator re-
sistance, for the bridge circuit of the sampling gate
2 1C C= .
The converted signal amplitude at the time in-
terval Θ is calculated by the formula:
( ) ( )
2
1
1 ,
t
H t
P i t dt
C
Θ = Θ∫ . (5)
Integration is performed in the time interval [ ]1 2,t t ,
when the bridge diodes are conducting. The charge
current of the storage capacitor HC composed of the
current in the sampling gate diodes and current aris-
ing owing to the signal passage through the diode
junction capacitance is described by the expression:
( ) ( ) ( )
( )
2
,, , dv ti t g t v t C
dt
ΘΘ = Θ + . (6)
If to transform the expression (5) taking into
account (6) we will obtain:
( ) CP Ucon UΘ = + (7)
where ( ) ( )
2
1
1
,
t
H t
Ucon g t v t dt
C
= Θ∫ is conditioned
by the diode conduction, and
( )2
1
2
1 ,
t
C
H t
dv tU C dt
C dt
Θ= ∫ is conditioned by the
signal passage through the junction capacitance.
As it follows from (7) the conversion is a sum of
two terms. The first term is Ucon conditioned by the
diode conduction. The second one is CU conditioned
by the signal passage through the junction capaci-
tance of the sampling gate diodes. It is CU that is the
cause of the waveform distortion during conversion
because the time dependence of the first term repeats
the time dependence of the converted signal, and the
second term depends on time as the time derivative
of the applied signal.
Fig. 2 shows the time dependence of each com-
ponent (7) in case that the input signal is the voltage
step with the amplitude 10 mV. In this case the con-
verted signal is a transient response of the sampler
( ) ( )U t P= Θ [6], which determines the sampler
speed and distortions of the converted signal. The
sampler speed is characterized by the rise time of the
transient response, which is 0.5 ns in the given case
and that indicates the possibility of signal conversion,
which rise (decay) time is more than 0.5 ns. It is
enough to convert georadar signals, the rise (decay)
time of which has a duration of the order of 1 ns.
To estimate distortions by the transient response
is possible by analyzing the contribution of Ucon
and CU . It is clearly seen from the figure that the
peak amplitude of the converted signal component
conditioned by the signal passage through the junc-
tion capacitance of the sampling gate diodes is at the
point in time 0.6 ns and reaches about a half of the
converted signal amplitude at this time.
After charging the junction capacitances of the
sampling gate diodes the charge current of these ca-
pacitances drops practically to zero. It happens about
1 ns after the signal arrival. Later on the diode cur-
rent is conditioned only by the diode conduction cur-
rent component. So the waveform distortions during
conversion are almost absent.
Fig. 3 shows the time dependence of the ratio
/CU Ucon . This dependence characterizes wave-
form distortions during conversion. The higher the
ratio /CU Ucon the bigger the capacitive current
contribution and the more significant the distortions.
As it is seen from the figure when applying the
voltage step to the sampler input the ratio /CU Ucon
Fig. 2. The converted voltage step with the ampli-
tude 10 mV; the dashed line shows a signal part
passed through the diode capacitance
The Sampler of the Videopulse Georadar
Radio Physics and Radio Astronomy, 2002, v. 7, No. 4 427
decreases rapidly and it is equal practically to zero
already at 0.9 ns. So waveform distortions during
conversion are observed in the time interval from 0 to
0.9 ns. Later on the distortions are negligible.
It is possible to shorten the time interval, where
distortions take place, by using diodes with lower
junction capacitance and also applying “servo” feed-
backs and enclosing by them the sampling gate and
buffer amplifier [1]. However, as it is seen from
Fig. 3, 1 ns after the signal arrival the contribution of
the junction capacitance is small. It means that the
influence of the diode junction capacitance can be
ignored for signals, which spectrum occupies the
bandwidth less than 1 GHz.
Thus, these calculations indicate the possibility
of creating the sampler, at which output the noise
level will be less than 1 mV and the conversion error
(taking into account the influence of stray parameters
of the sampling gate) at the bandwidth boundary will
be no more than 0,2 1u mV± .
3. The Sampler
It is necessary for signal conversion with the cutoff
frequency in the spectrum 1 GHz that the strobe
pulse duration at the level when the sampling gate
diodes become conducting will be no more than
0.25 ns [15]. To obtain these parameters we should
select carefully the operating mode of the strobe gen-
erator and set an appropriate voltage shift of the
bridge diodes.
3.1. An Optimum Operation Mode of the Strobe
Pulse Generator
The strobe generator operates in the following way.
The starting pulse with rise time of the order of 3 ns
and the amplitude 10 V is applied to the input of the
matching emitter repeater with the high-frequency
transistor VT1. From its output pulses comes through
the transformer to the diode peaker. The diode
КД524Б operating in the charge storage mode is used
in the diode peaker circuit as an active element.
Characteristics of КД524Б satisfy completely basic
requirements to the strobe generator diodes. This
diode has the low stray inductance and capacitance
and also the low direct resistance what permits to
form the current step of nanosecond duration.
A strobe pulse is formed by the short-circuited
line [6]. A part of the voltage step 1U (Fig. 4) passes
through the balanced transformer and forms the pulse
leading edge. Another part 2U passes through the
short-circuited line and after reflection it forms the
pulse droop. So, the short pulse is formed after pass-
ing through the long line.
To received a pulse with duration
0f dT T T+ + , at the shaper output it is necessary
that 0 2 /f uT T L v+ = , where uv is the electro-
magnetic wave velocity in the line, and L is the
length of the short-circuited line. For 0.25fT = ns,
0 0T = ns, 0.25dT = ns, /uv c ε= , and for
teflon 1,5ε = we obtain 2.5L = cm.
At the current step the charge-storage diode has
the low resistance [8, 11], and is badly matched with
the high-resistance load being the diode sampling
gate in this case.
The balanced broad-band transformer T2
(Fig. 1) with the input resistance 50 Ohms and output
resistance 20 Ohms was used to match resistances of
the strobe generator and sampling gate in the investi-
gated sampler. Besides the sampling gate diodes with
the lowest differential resistance were selected. In the
given case we succeeded in selecting a set of diodes
with a differential resistance of the order of
20 Ohms.The measurements show that the amplitude
of strobe pulses decreases with their duration. It is
conditioned on the one hand by attenuation in the
balanced line and on the other hand by the arrival
time of the reflected signal 2U . On condition that
Fig. 3. Dependence of the ratio /CU Ucon on time
Fig. 4. The strobe pulse forming in the short-
circuited line
P.V. Kholod, V.P. Ruban
428 Radio Physics and Radio Astronomy, 2002, v. 7, No. 4
0 0T = , the sooner the reflected signal arrives, the
sooner 1U combines with 2U . Therefore an input
pulse with the amplitude 10 V and duration 1 sµ is
needed to form a strobe pulse with the amplitude 1 V
and duration 0.25 ns at the level 0.5 maxU (Fig. 5).
The obtained pulse satisfies completely the re-
quirements to strobe pulses of the georadar sampler,
both in amplitude and duration.
3.2. Sampling Gate. Tuning and Testing.
The diodes D2-D5 of the КД514А type with the total
capacitance of each diode less than 0.9 pF were used
in the sampler made according to the balanced bridge
circuit. The diodes were selected in pairs in resis-
tance. The working point shift in the diode character-
istic is set by controlling the trimming resistors R7
and R10 of the voltage divider. The shift value de-
termines the sampling time that in its turn determines
the sampler working bandwidth.
Failing a signal at the sampling gate input the
diode bridge is balanced and the charge value of the
storage capacitance doesn’t change. At applying the
investigated signal to the sampler input the bridge
becomes unbalanced, and the capacitance С14 is
charged by the diode difference current proportional
to the signal amplitude at the input. When the strobe
pulse stops acting the capacitance С14 is discharged
through the preamplifier input resistance.
The sequence of R11, C13 and R12, C12 imi-
tates the sampling gate load at closed bridge diodes.
The voltage in them is applied to the buffer amplifier
input in the antiphase with the stray signal of the di-
rect leakage through diode capacitances. Thus, the
undesirable distortions are compensated.
In the sampling gate bridge circuit the distor-
tions during signal conversion are caused both by
stray parameters of the sampling gate diodes, such as:
junction capacitances, and by the bridge unbalance.
As a result of their “complex action” the sawtooth
voltage arises at the sampler output Fig. 6.
In order to eliminate this sawtooth voltage we
should select carefully the sampling gate diodes try-
ing for the identity of their parameters and tune the
cutoff bridge voltage. In that way it is possible to
decrease the sawtooth voltage amplitude down to
1 mV.
In the given sampler the stray sawtooth voltage
period is 40 sµ (the lower frequency limit in the
spectrum is 25 kHz). As the converted signal spec-
trum is from 0 to 20 kHz it became possible to filter
the signal. For that it is necessary to use the compen-
sating filter of low frequencies (Fig. 7).
The compensating filter is made with the opera-
tional amplifier КР140УД8А. The filter boundary
frequency is determined by the sequences R2, C1 and
R1, R2, C2. The application of this filter reduces the
amplitude of the stray sawtooth voltage by more than
10 dB. At that the converted signal shape replicates
the signal shape at the sampler input.
Fig. 5. The strobe pulse oscillogram with duration
0.25 ns, at the level max0.5 U
Fig. 6. Distortions:
1 – at complete bridge unbalancing;
2 – at diode selection;
3 – at the cutoff voltage tuning to 0.3 V
Fig. 7. The afterfilter circuit
The Sampler of the Videopulse Georadar
Radio Physics and Radio Astronomy, 2002, v. 7, No. 4 429
The comparative oscillograms of converted sig-
nals obtained by the georadar (right) and oscilloscope
С1-70 (left) are shown in Fig. 8.
As the figure shows the duration of each pair of
converted pulses is almost the same. Differences are
only in amplitude in the pulse midband width, that is
caused by the peculiarity of the low-frequency ampli-
fication path of georadar.
4. Conclusions
The shown in this work estimation of the influence of
the sampling gate and strobe generator parameters on
noise characteristics of the sampler indicates the pos-
sibility of constructing a sampler with sensitivity of
the order of 341 µV in the working frequency band-
width up to 1 GHz. Besides, it has been determined
that at the given stray parameters of the circuit ele-
ments the inaccuracy of sampler measurements at the
frequency upper boundary is 0,2 1u mV± .
The strobe generator used in the sampler with a
short-circuited line of 2.5 cm allowed us to obtain
strobe pulses with the 0.25 ns duration at the level
max0.5 U and amplitude 1 V. Moreover, the sam-
pling gate setting features related with cutoff diode
voltage installation. The cutoff voltage decreases
down to 0.3 V and that allowed us to decrease signal
distortions in the sampling gate conditioned by the
bridge unbalance, from 20 mV to 1 mV.
The distortion in the sampler caused by the non-
identity of the sampling gate diode parameters and
influence of the junction capacitance has been elimi-
nated by the compensating filter insertion.
The experimental check of calculation results
has shown that the sampler sensitivity is not lower
than 1 mV in the working bandwidth up to 1 GHz.
The sensitivity can be increased by using high-
identical diode pairs of the sampling gate and also by
applying servo feedbacks. The working bandwidth of
the sampler can be also expanded first of all by de-
creasing the junction capacitance of the sampling
gate diodes and also by decreasing their stray pa-
rameters: lead inductance and capacitance.
Control measurements have shown both the
amplitude and time linearity of signal conversion,
and that permits us to use the developed design of the
sampler in up-to-date videopulse radars for subsur-
face sounding.
Fig. 7. Comparison of signal conversion by samplers of the oscilloscope С1-70 (left column) and georadar
(right column)
P.V. Kholod, V.P. Ruban
430 Radio Physics and Radio Astronomy, 2002, v. 7, No. 4
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15. Technical brief. Delta-time accuracy: the good, the
bad, and the worst case. Tektronics. Inc, 2000, pp.4-5.
СТРОБОСКОПИЧЕСКИЙ
ПРЕОБРАЗОВАТЕЛЬ
ВИДЕОИМПУЛЬСНОГО ГЕОРАДАРА
В.П. Рубан, П.В. Холод
В работе представлен теоретический анализ шу-
мовых характеристик и искажений стробоскопического
преобразователя с полосой пропускания 1 ГГц и чувст-
вительностью 1 мВ, построенного по мостовой диод-
ной схеме. Приведены результаты тестовых измерений
макета преобразователя.
СТРОБОСКОПІЧНИЙ
ПЕРЕТВОРЮВАЧ
ВІДЕОІМПУЛЬСНОГО ГЕОРАДАРА
В.П. Рубан, П.В. Холод
У роботі представлений теоретичний аналіз шу-
мових характеристик та спотворень стробоскопічного
перетворювача з полосою пропускання 1 ГГц та чутли-
вістю 1 мВ, побудованого за мостовою діодною схе-
мою. Наведено результати тестових вимірювань, про-
ведених за допомогою макета перетворювача.
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